Refactor git descriptors

[mathbou]

Hi there,

I was looking at my temp folder while doing some tests with the tk-core, and I saw that each time I reload an engine, a massive amount of temporary clone folders were created (associated with a big network load). By digging into the source code of git descriptors, I found that they clone repos each and every time they needs to execute a command in it.

So here is a PR with a big refactoring of the git, git_tag and git_branch descriptors:

• replace git clone with git ls-remote when fetching tags/commit values, checking remote access
• Add version pattern and get_latest support for git branch, and improve the logic for git tag
• Add caching system to avoid processing the same repo multiple times

[codecov]

Codecov Report

Merging #913 (168e364) into master (e2e2173) will decrease coverage by 5.94%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #913      +/-   ##
==========================================
- Coverage   78.91%   72.98%   -5.94%     
==========================================
  Files         182      182              
  Lines       20083    20116      +33     
==========================================
- Hits        15849    14681    -1168     
- Misses       4234     5435    +1201     

Files Changed	Coverage Δ
python/tank/descriptor/io_descriptor/git.py	89.62% <87.77%> (+3.45%)	⬆️
python/tank/descriptor/io_descriptor/git_tag.py	95.09% <93.47%> (+3.33%)	⬆️
python/tank/descriptor/io_descriptor/git_branch.py	91.04% <96.29%> (+7.96%)	⬆️
python/tank/util/filesystem.py	86.58% <100.00%> (+0.55%)	⬆️

... and 47 files with indirect coverage changes

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